And I'm Robert Siegel. In just a few short hours, Iowa Republicans will travel to their precinct caucus sites around the state, and there, they will cast a ballot for the person they want to be their next president of the United States or for the person they think has the best chance of beating the current president, Barack Obama. NPR national political correspondent Mara Liasson joins us now. And, Mara, first of all, can you give us a quick reminder of just how the caucuses will work tonight?

MARA LIASSON, BYLINE: The caucuses begin at 7 o'clock sharp. Supporters of each candidate will arrive at their caucus sites with their scripts. They'll have talking points. They'll make the speeches about why their guy is the best guy. And then, it's pretty simple. Republican caucuses are essentiality a straw poll - one secret ballot, pieces of paper dropped in a box, and that's it.

SIEGEL: None of this eliminating the lowest candidate (unintelligible)?

LIASSON: Nope. Only Democrats do that.

SIEGEL: Who is most likely to show up at tonight's caucuses?

LIASSON: Well, I can tell you who showed up last time. In 2008, there were more men than women; 99 percent of the caucus-goers in the Republican Party were white; 1 percent Hispanic or Asian; no black Iowa Republicans; they're generally older; 73 percent were over 45; 60 percent in 2008 described themselves as born-again Christians or evangelicals. 120,000 people showed up four years ago, which is the bar against which Republicans are going to be held tonight.

We think a high turnout will probably help Mitt Romney; a low turnout will probably benefit Ron Paul. The weather doesn't seem to be any barrier in Iowa today. And Democrats don't have a competitive caucus on their side, so some Democrats and independents who voted Democratic last time might turn out for the Republican caucus, which they can do - they can register as Republicans on site. We also know there's been less retail campaigning this year, and the crowds have been smaller for everyone, even in these last days, compared to last year.

SIEGEL: Four years ago, Mitt Romney won 25 percent of the vote in Iowa, in the caucuses. And he's polling just around 25 percent of the vote. Now, this is - it's a magic number for the Romney campaign.

LIASSON: He's the 25 percent man.

SIEGEL: Yeah.

LIASSON: That's right. And it says that 75 percent of Republicans don't want him. I think the conservative grassroots resistance to Romney has been one of the signal features of this campaign. It's been constant, unchanging. One conservative commentator said, today, Mitt Romney is like Kerry without the medals, but conservatives never found an alternative. They tried out each one. There was one surge after another, and none of them caught on. It is worth noting that this was a particularly weak field.

The superstars in the Republican Party stayed home - Chris Christie, Paul Ryan, Mitch Daniels, Jeb Bush. And in the end, Republicans may revert to form and fall in line behind someone instead of falling in love with them. But you're right; it raises the question: Will there be a lack of enthusiasm among of Republicans if Romney is the nominee in the general election, or will the desire to defeat Barack Obama overcome that?

SIEGEL: Mara, a year ago, we were marking the beginning of the age of the Tea Party. How important is the Tea Party in the Republican Party in Iowa this year?

LIASSON: Well, it is very important. It's just not monolithic, and they never coalesced around one anti-Romney candidate, even though they did move the party to the right. Rick Perry was the only one who could have given Romney a real challenge, but he stumbled out of the gate and never got up. What it means, though, is that Mitt Romney was never really challenged. He never had a barrage of attack ads aimed at him the way Newt Gingrich did. And that's remarkable considering that he is the most moderate and the least Tea Party-ish candidate in the race.

SIEGEL: Finally, President Obama, the White House says he doesn't plan to watch the returns, maybe he'll listen. But will he be talking with - he will talk with Democratic caucus-goers tonight.

LIASSON: Yes, he is. The Democrats are treating Iowa as an organized exercise. They want Democrats to come to their caucuses tonight. It's a dry-run for the general election. They've made hundreds of thousands of phone calls. They're trying to reactivate the grassroots army they created there in 2007 and '08, and that is for one big important reason: Iowa is a swing state.